1. Field of the Invention
The present invention relates to a film resistor heater comprising a sprayed film resistor comprising NiCr particles uniformly dispersed in an insulating matrix.
2. Description of the Prior Art
Sheathed heaters have conventionally been used for the purpose of heating various objects. A typical sheathed heater comprises an aluminum sheath, an MgO insulating powder contained in the sheath and an NiCr wire embedded in the insulating powder. When a plate or a vessel is to be heated, the sheathed heater is attached to the wall of the plate or the vessel by caulking, etc. Since the sheathed heater is round in cross-section, its contact area with the wall is very small. Thus, heat directly conducted from the sheathed heater to the wall via the above contact area is inevitably small. In addition, if the sheathed heater is placed in a vacuum atmosphere such as in a vacuum kettle, a small gap which inevitably exists between the sheathed heater and the wall makes it hard to transmit the heat generated by the sheathed heater to the wall efficiently. Therefore, sheathed heaters are disadvantageous because of their limited heat transmission efficiency.
Ceramic resistor heaters have recently been developed. Mr. Tamamizu disclosed in his article "Ceramic Resistor Heater," Electronic Ceramics, Vol. 6 (No. 40) 66-71 (1980), various sintered ceramics such as SiC, MoSi.sub.2, LaCrO.sub.3 and ZrO.sub.2 which may be used as heat-generating bodies. These sintered ceramic heaters are used primarily for heating furnaces to temperatures of 1600.degree. C.-2000.degree. C. If these sintered ceramic heaters are used for heating plates and vessels, they have to be attached to the walls of plates and vessels. In this case, too, complete contact of these sintered ceramic heaters with the walls cannot be achieved.
Attempts have been made to form heat-generating ceramic films on substrates by spraying, particularly plasma spraying. Smyth et al. disclosed the production of NiO Fe.sub.3 O.sub.4 ceramic resistors by arc plasma spraying in "Production of Resistors by Arc Plasma Spraying," Electrocomponent Science and Technology, Vol. 2, 135-145 (1975). The NiO Fe.sub.3 O.sub.4 ceramic resistors, however, have resistivity which varies sharply as the ratio of NiO to Fe.sub.3 O.sub.4 changes. Therefore, the production of NiO.Fe.sub.3 O.sub.4 ceramic resistors having the desired resistivity requires strict control of the composition of a NiO.Fe.sub.3 O.sub.4 mixture.
Japanese Patent Laid-Open No. 59-130080 discloses the plasma spraying of TiO.sub.2 powder to form a resistor on an insulator-coated plate. TiO.sub.2 is reduced to TiO.sub.2-x during the plasma spraying in an atmosphere of argon and hydrogen. The TiO.sub.2-x film resistor, however, has resistivity which lowers drastically as the temperature is elevated near room temperature and is very low when the temperature is high. Accordingly, it is difficult to have the desired resistivity during the overall heating operation.